[v4] mm, vmscan: retry kswapd's priority loop with cache_trim_mode off on failure

Commit Message

Byungchul Park March 4, 2024, 2:09 a.m. UTC

Changes from v3:
	1. Update the test result in the commit message with v4.
	2. Retry the whole priority loop with cache_trim_mode off again,
	   rather than forcing the mode off at the highest priority,
	   when the mode doesn't work. (feedbacked by Johannes Weiner)

Changes from v2:
	1. Change the condition to stop cache_trim_mode.

	   From - Stop it if it's at high scan priorities, 0 or 1.
	   To   - Stop it if it's at high scan priorities, 0 or 1, and
	          the mode didn't work in the previous turn.

	   (feedbacked by Huang Ying)

	2. Change the test result in the commit message after testing
	   with the new logic.

Changes from v1:
	1. Add a comment describing why this change is necessary in code
	   and rewrite the commit message with how to reproduce and what
	   the result is using vmstat. (feedbacked by Andrew Morton and
	   Yu Zhao)
	2. Change the condition to avoid cache_trim_mode from
	   'sc->priority != 1' to 'sc->priority > 1' to reflect cases
	   where the priority goes to zero all the way. (feedbacked by
	   Yu Zhao)

--->8---
From bd275d8d8e9ed91869401f0e1d089c4a2aeeaac6 Mon Sep 17 00:00:00 2001
From: Byungchul Park <[e-mail]>
Date: Thu, 29 Feb 2024 19:48:01 +0900
Subject: [PATCH v4] mm, vmscan: retry kswapd's priority loop with cache_trim_mode off on failure

With cache_trim_mode on, reclaim logic doesn't bother reclaiming anon
pages.  However, it should be more careful to use the mode because it's
going to prevent anon pages from being reclaimed even if there are a
huge number of anon pages that are cold and should be reclaimed.  Even
worse, that leads kswapd_failures to reach MAX_RECLAIM_RETRIES and
stopping kswapd from functioning until direct reclaim eventually works
to resume kswapd.

So kswapd needs to retry its scan priority loop with cache_trim_mode
off again if the mode doesn't work for reclaim.

The problematic behavior can be reproduced by:

   CONFIG_NUMA_BALANCING enabled
   sysctl_numa_balancing_mode set to NUMA_BALANCING_MEMORY_TIERING
   numa node0 (8GB local memory, 16 CPUs)
   numa node1 (8GB slow tier memory, no CPUs)

   Sequence:

   1) echo 3 > /proc/sys/vm/drop_caches
   2) To emulate the system with full of cold memory in local DRAM, run
      the following dummy program and never touch the region:

         mmap(0, 8 * 1024 * 1024 * 1024, PROT_READ | PROT_WRITE,
              MAP_ANONYMOUS | MAP_PRIVATE | MAP_POPULATE, -1, 0);

   3) Run any memory intensive work e.g. XSBench.
   4) Check if numa balancing is working e.i. promotion/demotion.
   5) Iterate 1) ~ 4) until numa balancing stops.

With this, you could see that promotion/demotion are not working because
kswapd has stopped due to ->kswapd_failures >= MAX_RECLAIM_RETRIES.

Interesting vmstat delta's differences between before and after are like:

   +-----------------------+-------------------------------+
   | interesting vmstat    | before        | after         |
   +-----------------------+-------------------------------+
   | nr_inactive_anon      | 321935        | 1646193       |
   | nr_active_anon        | 1780700       | 456388        |
   | nr_inactive_file      | 30425         | 27836         |
   | nr_active_file        | 14961         | 1217          |
   | pgpromote_success     | 356           | 1310120       |
   | pgpromote_candidate   | 21953245      | 1736872       |
   | pgactivate            | 1844523       | 3292443       |
   | pgdeactivate          | 50634         | 1526701       |
   | pgfault               | 31100294      | 6715375       |
   | pgdemote_kswapd       | 30856         | 1954199       |
   | pgscan_kswapd         | 1861981       | 7100099       |
   | pgscan_anon           | 1822930       | 7061135       |
   | pgscan_file           | 39051         | 38964         |
   | pgsteal_anon          | 386           | 1925214       |
   | pgsteal_file          | 30470         | 28985         |
   | pageoutrun            | 30            | 500           |
   | numa_hint_faults      | 27418279      | 3090773       |
   | numa_pages_migrated   | 356           | 1310120       |
   +-----------------------+-------------------------------+

Signed-off-by: Byungchul Park <[e-mail]>
---
 mm/vmscan.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

Patch

diff --git a/mm/vmscan.c b/mm/vmscan.c
index bba207f41b14..7ec1fbcba7d4 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -108,6 +108,9 @@  struct scan_control {
 	/* Can folios be swapped as part of reclaim? */
 	unsigned int may_swap:1;
 
+	/* Can cache_trim_mode be turned on as part of reclaim? */
+	unsigned int may_cache_trim_mode:1;
+
 	/* Proactive reclaim invoked by userspace through memory.reclaim */
 	unsigned int proactive:1;
 
@@ -2268,7 +2271,8 @@  static void prepare_scan_control(pg_data_t *pgdat, struct scan_control *sc)
 	 * anonymous pages.
 	 */
 	file = lruvec_page_state(target_lruvec, NR_INACTIVE_FILE);
-	if (file >> sc->priority && !(sc->may_deactivate & DEACTIVATE_FILE))
+	if (file >> sc->priority && !(sc->may_deactivate & DEACTIVATE_FILE) &&
+	    sc->may_cache_trim_mode)
 		sc->cache_trim_mode = 1;
 	else
 		sc->cache_trim_mode = 0;
@@ -6744,6 +6748,7 @@  static int balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)
 		.gfp_mask = GFP_KERNEL,
 		.order = order,
 		.may_unmap = 1,
+		.may_cache_trim_mode = 1,
 	};
 
 	set_task_reclaim_state(current, &sc.reclaim_state);
@@ -6898,8 +6903,14 @@  static int balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)
 			sc.priority--;
 	} while (sc.priority >= 1);
 
-	if (!sc.nr_reclaimed)
+	if (!sc.nr_reclaimed) {
+		if (sc.may_cache_trim_mode) {
+			sc.may_cache_trim_mode = 0;
+			goto restart;
+		}
+
 		pgdat->kswapd_failures++;
+	}
 
 out:
 	clear_reclaim_active(pgdat, highest_zoneidx);